|Publication number||US6358118 B1|
|Application number||US 09/608,462|
|Publication date||19 Mar 2002|
|Filing date||30 Jun 2000|
|Priority date||30 Jun 2000|
|Also published as||US6612904|
|Publication number||09608462, 608462, US 6358118 B1, US 6358118B1, US-B1-6358118, US6358118 B1, US6358118B1|
|Inventors||Robert G. Boehm, John M. Boyd|
|Original Assignee||Lam Research Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (7), Non-Patent Citations (1), Referenced by (68), Classifications (16), Legal Events (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This invention relates to the fabrication of integrated circuits, and more particularly, to a manufacturing apparatus and a method that planarizes wafer surfaces.
The fabrication of integrated circuits involves a sequence of steps. The process can involve the deposition of thin films, the patterning of features, the etching of layers, and the polishing of surfaces to planarize or remove contaminants.
Chemical Mechanical Polishing (“CMP”) is one process that planarizes surfaces and removes contaminants. A CMP process involves subjecting a semiconductor wafer to a rotating pad and a chemical slurry. The polishing process is a grinding of the wafer surface and a chemical reaction between the surface and the chemical slurry.
Planarizing and cleaning wafer surfaces by a CMP process can be very effective but also can be difficult to control. Removal rates by a CMP process can change with the rotation rates of the pad and the wafer, by the pH or flow rates of the chemical slurry, or by the distribution of the chemical slurry near the center of the wafer, for example. Even variations in feature densities or pressure variations across the polishing pad can cause variations in the removal rates of wafer layers and contaminants.
Controlling the removal rates can be a very difficult process given that many other parameters can also cause variations. Accordingly, there is a need to control the removal rates across an entire or a selected portion of a wafer surface.
FIG. 1 is a perspective view of a preferred embodiment.
FIG. 2 is a cross-sectional view of FIG. 1.
FIG. 3 is a partial cross-sectional view of FIG. 1.
FIG. 4 is a cross-sectional view of an alternative preferred embodiment incorporated in a rotary tool.
FIG. 5 is a partial cross-sectional view of FIG. 4.
FIG. 6 is a partial top view of a platen and magnetic fields of FIG. 5.
Embodiments of the apparatus and method of the present invention discussed below provide significant improvements for controlling surface removal rates and polishing profiles by a CMP or a silicon polishing process. The apparatus and the method utilize force modulation to control these rates across an entire or a selected portion of a wafer surface. The apparatus and the method substantially eliminate surface variations between the center, middle, and edge regions of a semiconductor wafer surface that can occur in CMP or silicon polishing processes.
FIG. 1 illustrates a perspective view of a preferred embodiment of the invention. The apparatus 2 preferably employs a belt 4 that moves linearly with respect to a semiconductor wafer 6. The belt 4 travels over rollers 8 that are driven in rotation by a motor or any other device that imparts a linear motion to the belt 4 with respect to the semiconductor wafer 6. A polishing pad 10 is affixed to the outer surface of the belt 4 and makes contact with the wafer surface.
The belt 4 is supported, in part, by a hollow fluid filled structure that serves as a receptacle for a powder, a fluid, or a gas. The hollow structure or bladder 12 provides support to the underside of the belt 4 against downward forces that press against the polishing pad 10 and the belt 4. A stiff polymer support or platen 14 disposed on the underside of the bladder 12 supports the bladder 12 against movement away from the belt 4. Beneath the pad 10 are flux guides that are connected to one or more Direct Current (“DC”) or Alternating Current (“AC” ) power supply/supplies 26 shown in FIG. 2. The flux guides are used to either direct a field or a magnetic flux to selected locations of the bladder 12 or prevent a field or a magnetic flux from reaching selected regions of the bladder 12.
The semiconductor wafer 6, which may be comprised of silicon scaled to the dimensions of a given circuit, is retained by a wafer carrier 16 enclosed by a housing 18. The semiconductor wafer 6 is held in place by a retention device and/or by a vacuum. In this preferred embodiment, the wafer 6 is rotated with respect to the belt 4 by the orbit of the wafer carrier 16. The rotation of the wafer 6 distributes contact between the pad 10 and the wafer 6 when the wafer 6 is pressed against the belt 4. The rotation of the wafer 6 allows for a substantially uniform removal rate or polishing profile of the wafer surface.
As shown in FIGS. 1 and 2, a dispensing member 20 is positioned above the pad 10 to dispense a chemical slurry 28 to an outer surface of the pad 10. The chemical slurry 28 can be a mixture of solid particles and liquid such as a colloidal silica and a pH-controlled liquid. Of course, other chemical slurry materials can also be used.
Other details of this preferred embodiment can be found in U.S. Pat. No. 5,916,012 entitled “Control of Chemical-Mechanical Polishing Rate Across a Substrate Surface for a Linear Polisher” assigned to the assignee of this invention. This patent is hereby incorporated by reference in its entirety.
The apparatus and method of this preferred embodiment further includes a material or a fluid means having a variable magnetic flux density or a variable viscosity such as a magnetic fluid 22. The magnetic fluid 22 is held within the bladder 12. Examples of such magnetic fluid 22 include a mixture of oil and ferromagnetic shavings, iron filings and gunk (i.e. a greasy substance), magneto-rheological fluid, or magnarheological fluid, for example. The magnetic fluid 12 functions like an active suspension system that compensates for CMP or silicon polishing process variations caused by parameter variances such as wafer surface irregularities, belt sag, linear belt rotation rates, slurry flow rates, device pattern densities, pitch areas, and wafer rotation rates, for example. The magnetic fluid 22 can compensate for these and many other process parameters that cause variation in the polishing profiles of the wafer layers. The magnetic fluid 22 also provides the necessary counteracting forces against the wafer 6 when the wafer carrier 16 presses the wafer 6 against the polishing pad 10.
Referring to FIG. 3, a partial cross-sectional view of this preferred embodiment is shown. Beneath the wafer 6 is the polishing pad 10 disposed on the belt 4. The pad 10 and the belt 4 move in a linear direction with respect to the wafer 6. Preferably, a device or feature side of the wafer 6 is positioned above the polishing pad 10. A stationary bladder 12, preferably made of a gasket or a flexible membrane material throughout, underlies the belt 4 to counteract or dampen downward forces. Besides having a low resistance to the linear motion of the belt 4, the bladder 12 preferably has other attributes including resistance to puncture, durability, a high resistance to wear, and a low magnetic flux resistivity. In this preferred embodiment, a synthetic resin such as polytetrafluoroethylene or Teflon coats the outer surface of the bladder 12 that underlies the belt 4. Preferably, the synthetic resin is not vulnerable to attack by a variety of chemicals, retains its physical properties over a wide temperature range, and has a low coefficient of friction.
As shown, a plurality of coils 24 are positioned below the bladder 12. In this preferred embodiment, the coils 24 are DC coils that serve as flux guides to direct an electric field, a magnetic field, an electromagnetic field, or a magnetic flux to selected locations of the bladder 12. The DC coils 24 illustrated in FIGS. 1-3 and FIG. 5 preferably generate uniform or differential fields that pass through the magnetic fluid 22 enclosed by the bladder 12. As the fields pass through portions of the magnetic fluid 22, those portions of the magnetic fluid 22 change viscosity and prevent some of the magnetic fluid 22 from flowing to sections of the bladder 12. The strength of the magnetic fluid's 22 resistance to flow is directly proportional to the rate of change of the field and/or the strength of the field. As the strength of the field increases, the magnetic density of the magnetic fluid 22 increases, which makes a smaller volume of the magnetic fluid 22 available to transfer the motion of a downward and/or a lateral force to other volumes of the magnetic fluid 22. By altering the viscosity of selected portions of the magnetic fluid 22, the apparatus and method of this preferred embodiment can generate many desired pressure profiles in support of the underside of the belt 4 and the polishing pad 10 and thus compensate for many polishing and grinding process parameters that cause polishing profile variations.
The degree of control and adjustment available to this preferred embodiment of the invention depends on a number of factors including, for example, the linear speed of the belt 4, the rotational speed of the wafer 6, the alignment of the wafer 6 and the polishing pad 10, the position of the flux guides, the shape of the flux guides, and the strength of the fields emanating from the flux guides. In the preferred embodiment illustrated in FIG. 3, the flux guides are coils 24 that have a substantially circular cross-section and are positioned across a width of the bladder 12. Preferably, the flux guides shapes and sizes emanate the desired field intensity to the desired locations. It should be noted, however, that flux guides are not limited to the illustrated dimensions, lengths, or the cross-sections of the coils 24 shown in the accompanying figures. Thus, the substantially circular cross-sectional shapes of the coils 24, their positions across the width of the bladder 12, and their illustrated diameters, illustrate only a few of the many forms that this aspect of the invention can take. The coils 24, for example, can have a polygonal cross-section and/or be positioned across the entire or a portion of the width or the length of the bladder 12.
In the embodiment shown in FIG. 3, the magnetic flux density or viscosity of selected portions of the magnetic fluid 22 is independently controlled by controlling the field emanating from one or more coils 24 adjacent to the selected portions of the fluid 22. This control provides a spatially controllable support for the polishing process. In use, the field emanating from the coils 24 can also overlap and thus provide a substantially uniform controllable support.
One or more power supplies 26 provide the desired DC current separately or collectively to the coils 24 shown in FIG. 2. In this preferred embodiment, the power supplies 26 are designed to the requirements of the polishing and grinding application. It should be understood that the type (i.e. manual or programmable) and the number of power supplies used in this preferred embodiment depend on the application and that a controller, such as a processor for example, can control the level of current flowing through each coil 24 separately or collectively and thus control the field(s) radiating through selected portions of the magnetic fluid 22.
Given that the polishing profile of a wafer surface is achieved by directing field(s) to selected locations of the bladder 12, the invention encompasses any structure that achieves that function. For example, the flux guides are not limited to current controlled coils 24 or even magnets. In alternative preferred embodiments, the flux guides can be electrodes positioned along the surface of the bladder 12, for example. Simply by passing current through selected electrodes and through selected portions of the magnetic fluid 22, the viscosity of the magnetic fluid 22 changes, which creates desired pressure profiles in support of the belt 4 and polishing pad 10 and creates the desired polishing profile(s) of the wafer 6. Likewise, the fluid encompasses any material in any physical state (i.e. solid, liquid, or gas) that can change mechanical properties when exposed to a magnetic field, an electromagnetic field, or a magnetic flux.
Furthermore, although many of the preferred embodiments have been described in reference to a linear polishing apparatus and method, they can be readily adapted to any polishing apparatus and method. For example, circular polishing tools or tools designed to the contour of the wafer 6 or any other material can be provided with the above described spatially controllable modulated force(s).
In yet another alternative embodiment, the apparatus and method of the invention can be adapted to a rotary polishing tool and/or an orbital system. In a preferred embodiment shown in FIGS. 4 and 5, a rotary polishing tool 30 includes an annular shaped bladder 12 supported by a rotary platen 32. The center of the bladder 12 is positioned about an axis 34 substantially coincident with a rotational axis 36 of the rotary platen 32. Coils 24 are disposed underneath the bladder 12 such that the coils 24 generate radially symmetrical magnetic fields 38, 40, and 42 that are substantially centered about axis 36 as shown in FIG. 6. It should be noted that the coils 24 are not limited to an annular shape or the illustrated annular cross-sections, diameters, or dimensions shown in FIG. 5 as this aspect of the invention can take many other forms. A few examples of rotary and orbital tools that can incorporate the invention include the Mirra Ebara 222™ by Applied Materials, the Auriga C™ by SpeedFam-IPEC and the 776 ™ by Orbital Systems. Of course, other tools including other rotary and orbital tools can also incorporate the invention.
From the forgoing description, it should be apparent that a wafer surface without circuitry or features, such as a pure silicon surface or layer for example, may be polished by the invention. Also, it should be apparent that the bladder 12 is not limited to any shape or dimension. FIGS. 1-5 illustrate only a few of the many shapes and dimensions the bladder 12 can take.
The field or magnetic flux control described above provides a number of advantages to the grinding and polishing of surfaces. By using fields or magnetic flux in a CMP or a wafer polishing apparatus and method, for example, there is no risk of contamination to the chemical slurry 28 or polishing process. The number of flux guides and their positions can be modified as desired, improving process control and reducing set-up times. The field or magnetic flux-control apparatus and method lends itself to open loop, closed loop, and automated control making it readily adaptable to many fabrication processes and facilities. The flux guides are highly reliable and further provide precise control of polishing profiles of an entire or a selected portion of a wafer surface.
The foregoing detailed description describes only a few of the many forms that the present invention can take and should therefore be taken as illustrative rather than limiting. It is only the following claims, including all equivalents that are intended to define the scope of the invention.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US5558568||2 Nov 1994||24 Sep 1996||Ontrak Systems, Inc.||Wafer polishing machine with fluid bearings|
|US5916012||25 Jun 1997||29 Jun 1999||Lam Research Corporation||Control of chemical-mechanical polishing rate across a substrate surface for a linear polisher|
|US5931718 *||30 Sep 1997||3 Aug 1999||The Board Of Regents Of Oklahoma State University||Magnetic float polishing processes and materials therefor|
|US5961372 *||5 Dec 1995||5 Oct 1999||Applied Materials, Inc.||Substrate belt polisher|
|US5980368 *||5 Nov 1997||9 Nov 1999||Aplex Group||Polishing tool having a sealed fluid chamber for support of polishing pad|
|US6086456 *||6 Nov 1998||11 Jul 2000||Aplex, Inc.||Polishing method using a hydrostatic fluid bearing support having fluctuating fluid flow|
|US6146245 *||6 May 1999||14 Nov 2000||Scientific Manufacturing Technologies, Inc.||Method of and device for machining flat parts|
|1||Peter Godwin, "The Car That Can't Crash", The New York Times Magazine, pp. 58-60, Jun. 11, 2000.|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US6540595 *||29 Aug 2000||1 Apr 2003||Applied Materials, Inc.||Chemical-Mechanical polishing apparatus and method utilizing an advanceable polishing sheet|
|US6561871 *||13 Jun 2000||13 May 2003||Applied Materials, Inc.||Linear drive system for chemical mechanical polishing|
|US6592437 *||26 Dec 2001||15 Jul 2003||Lam Research Corporation||Active gimbal ring with internal gel and methods for making same|
|US6607425 *||21 Dec 2000||19 Aug 2003||Lam Research Corporation||Pressurized membrane platen design for improving performance in CMP applications|
|US6612904 *||27 Dec 2001||2 Sep 2003||Lam Research Corporation||Field controlled polishing apparatus|
|US6656024 *||21 Dec 2001||2 Dec 2003||Lam Research Corporation||Method and apparatus for reducing compressed dry air usage during chemical mechanical planarization|
|US6729945 *||30 Mar 2001||4 May 2004||Lam Research Corporation||Apparatus for controlling leading edge and trailing edge polishing|
|US6767428 *||20 Dec 2001||27 Jul 2004||Lam Research Corporation||Method and apparatus for chemical mechanical planarization|
|US6848970 *||16 Sep 2002||1 Feb 2005||Applied Materials, Inc.||Process control in electrochemically assisted planarization|
|US6939212||21 Dec 2001||6 Sep 2005||Lam Research Corporation||Porous material air bearing platen for chemical mechanical planarization|
|US6955588||31 Mar 2004||18 Oct 2005||Lam Research Corporation||Method of and platen for controlling removal rate characteristics in chemical mechanical planarization|
|US6991526||16 Sep 2002||31 Jan 2006||Applied Materials, Inc.||Control of removal profile in electrochemically assisted CMP|
|US7018273||27 Jun 2003||28 Mar 2006||Lam Research Corporation||Platen with diaphragm and method for optimizing wafer polishing|
|US7033250 *||30 Jun 2004||25 Apr 2006||Lam Research Corporation||Method for chemical mechanical planarization|
|US7070475||1 Feb 2005||4 Jul 2006||Applied Materials||Process control in electrochemically assisted planarization|
|US7112270||6 Jun 2003||26 Sep 2006||Applied Materials, Inc.||Algorithm for real-time process control of electro-polishing|
|US7422982||7 Jul 2006||9 Sep 2008||Applied Materials, Inc.||Method and apparatus for electroprocessing a substrate with edge profile control|
|US7431634 *||6 Feb 2007||7 Oct 2008||Samsung Electronics, Co., Ltd.||Platen assembly, apparatus having the platen assembly and method of polishing a wafer using the platen assembly|
|US7628905||27 Jun 2006||8 Dec 2009||Applied Materials, Inc.||Algorithm for real-time process control of electro-polishing|
|US7655565||26 Jan 2005||2 Feb 2010||Applied Materials, Inc.||Electroprocessing profile control|
|US7709382||23 Oct 2007||4 May 2010||Applied Materials, Inc.||Electroprocessing profile control|
|US7790015||31 Oct 2007||7 Sep 2010||Applied Materials, Inc.||Endpoint for electroprocessing|
|US7842169||4 Mar 2003||30 Nov 2010||Applied Materials, Inc.||Method and apparatus for local polishing control|
|US8038509||7 Aug 2006||18 Oct 2011||Seung-Hun Bae||Chemical mechanical polishing apparatus|
|US8075703||10 Dec 2009||13 Dec 2011||Lam Research Corporation||Immersive oxidation and etching process for cleaning silicon electrodes|
|US8550880||10 Dec 2009||8 Oct 2013||Lam Research Corporation||Platen and adapter assemblies for facilitating silicon electrode polishing|
|US9102030 *||27 Jun 2011||11 Aug 2015||Corning Incorporated||Edge finishing apparatus|
|US9120201||9 Sep 2013||1 Sep 2015||Lam Research Corporation||Platen and adapter assemblies for facilitating silicon electrode polishing|
|US20040053512 *||16 Sep 2002||18 Mar 2004||Applied Materials, Inc.||Process control in electrochemically assisted planarization|
|US20040053560 *||16 Sep 2002||18 Mar 2004||Lizhong Sun||Control of removal profile in electrochemically assisted CMP|
|US20040072445 *||30 Jun 2003||15 Apr 2004||Applied Materials, Inc.||Effective method to improve surface finish in electrochemically assisted CMP|
|US20040173461 *||4 Mar 2003||9 Sep 2004||Applied Materials, Inc.||Method and apparatus for local polishing control|
|US20040182721 *||18 Mar 2003||23 Sep 2004||Applied Materials, Inc.||Process control in electro-chemical mechanical polishing|
|US20040214514 *||28 Apr 2003||28 Oct 2004||Elledge Jason B.||Polishing machines including under-pads and methods for mechanical and/or chemical-mechanical polishing of microfeature workpieces|
|US20040238493 *||30 Jun 2004||2 Dec 2004||Lam Research Corporation||Method for chemical mechanical planarization|
|US20050061674 *||24 Sep 2004||24 Mar 2005||Yan Wang||Endpoint compensation in electroprocessing|
|US20050121141 *||12 Nov 2004||9 Jun 2005||Manens Antoine P.||Real time process control for a polishing process|
|US20050178743 *||1 Feb 2005||18 Aug 2005||Applied Materials, Inc.||Process control in electrochemically assisted planarization|
|US20050233578 *||16 May 2005||20 Oct 2005||Applied Materials, Inc.||Method and composition for polishing a substrate|
|US20060021974 *||24 Sep 2004||2 Feb 2006||Applied Materials, Inc.||Method and composition for polishing a substrate|
|US20060057812 *||14 Sep 2004||16 Mar 2006||Applied Materials, Inc.||Full sequence metal and barrier layer electrochemical mechanical processing|
|US20060163074 *||6 Jun 2003||27 Jul 2006||Applied Materials, Inc.||Algorithm for real-time process control of electro-polishing|
|US20060166500 *||26 Jan 2005||27 Jul 2006||Applied Materials, Inc.||Electroprocessing profile control|
|US20060169674 *||26 Jan 2006||3 Aug 2006||Daxin Mao||Method and composition for polishing a substrate|
|US20060196778 *||27 Jan 2006||7 Sep 2006||Renhe Jia||Tungsten electroprocessing|
|US20060219663 *||23 Jan 2006||5 Oct 2006||Applied Materials, Inc.||Metal CMP process on one or more polishing stations using slurries with oxidizers|
|US20060228992 *||20 Jun 2006||12 Oct 2006||Manens Antoine P||Process control in electrochemically assisted planarization|
|US20060237330 *||27 Jun 2006||26 Oct 2006||Applied Materials, Inc.||Algorithm for real-time process control of electro-polishing|
|US20060252349 *||5 Aug 2005||9 Nov 2006||Yoo Jae H||Semiconductor wafer polishing apparatus having magneto-rhelogical elastic pad|
|US20060260951 *||21 Jun 2006||23 Nov 2006||Liu Feng Q||Full Sequence Metal and Barrier Layer Electrochemical Mechanical Processing|
|US20060286906 *||21 Jun 2005||21 Dec 2006||Cabot Microelectronics Corporation||Polishing pad comprising magnetically sensitive particles and method for the use thereof|
|US20070184759 *||6 Feb 2007||9 Aug 2007||Samsung Electronics Co., Ltd.||Platen assembly, apparatus having the platen assembly and method of polishing a wafer using the platen assembly|
|US20070218587 *||7 Mar 2007||20 Sep 2007||Applied Materials, Inc.||Soft conductive polymer processing pad and method for fabricating the same|
|US20080014709 *||7 Jul 2006||17 Jan 2008||Applied Materials, Inc.||Method and apparatus for electroprocessing a substrate with edge profile control|
|US20080017521 *||26 Jul 2007||24 Jan 2008||Manens Antoine P||Process control in electro-chemical mechanical polishing|
|US20080035474 *||23 Oct 2007||14 Feb 2008||You Wang||Apparatus for electroprocessing a substrate with edge profile control|
|US20080045012 *||23 Oct 2007||21 Feb 2008||Manens Antoine P||Electroprocessing profile control|
|US20080047841 *||23 Oct 2007||28 Feb 2008||Manens Antoine P||Electroprocessing profile control|
|US20080051009 *||31 Oct 2007||28 Feb 2008||Yan Wang||Endpoint for electroprocessing|
|US20090008600 *||24 Oct 2007||8 Jan 2009||Renhe Jia||Method and composition for polishing a substrate|
|US20090093193 *||7 Aug 2006||9 Apr 2009||Seung-Hun Bae||Chemical mechanical polishing apparatus|
|US20100139692 *||10 Dec 2009||10 Jun 2010||Lam Research Corporation||Immersive oxidation and etching process for cleaning silicon electrodes|
|US20100144246 *||10 Dec 2009||10 Jun 2010||Lam Research Corporation||Platen and adapter assemblies for facilitating silicon electrode polishing|
|US20120009854 *||27 Jun 2011||12 Jan 2012||Charles Michael Darcangelo||Edge finishing apparatus|
|US20120171933 *||13 Dec 2011||5 Jul 2012||Applied Materials, Inc.||Pressure controlled polishing platen|
|CN105458839A *||18 Nov 2015||6 Apr 2016||宇环数控机床股份有限公司||Magnetorheological polishing method and device|
|WO2004098832A1 *||26 Apr 2004||18 Nov 2004||Micron Technology, Inc.||Polishing machines including under-pads and methods for mechanical and/or chemical-mechanical polishing of microfeature workpieces|
|WO2007018391A1 *||7 Aug 2006||15 Feb 2007||Seung-Hun Bae||Chemical mechanical polishing apparatus|
|U.S. Classification||451/24, 451/307, 451/303, 451/41|
|International Classification||B24B37/20, B24B49/16, B24D9/08, B24B1/00|
|Cooperative Classification||B24D9/08, B24B49/16, B24B1/005, B24B37/20|
|European Classification||B24B37/20, B24B49/16, B24D9/08, B24B1/00D|
|19 Oct 2000||AS||Assignment|
Owner name: LAM RESEARCH, CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:BOEHM, ROBERT G.;BOYD, JOHN M.;REEL/FRAME:011175/0226;SIGNING DATES FROM 20000921 TO 20001009
|19 Sep 2005||FPAY||Fee payment|
Year of fee payment: 4
|18 May 2008||AS||Assignment|
Owner name: APPLIED MATERIALS, INC., CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:LAM RESEARCH CORPORATION;REEL/FRAME:020951/0935
Effective date: 20080108
|26 Oct 2009||REMI||Maintenance fee reminder mailed|
|19 Mar 2010||LAPS||Lapse for failure to pay maintenance fees|
|11 May 2010||FP||Expired due to failure to pay maintenance fee|
Effective date: 20100319